The present invention relates to frequency modulation (FM) demodulating apparatus in general and in particular to FM demodulating apparatus comprising an automatic threshold signal controlling circuit for use in an audio frequency Doppler shifted ultrasound blood circulation measuring apparatus.
Ultrasound Doppler instruments are presently used for measuring the velocity of blood in a body. To eliminate the effects of noise, such as instrument-induced noise, from the output signals, it has been the practice to provide on the instruments an operator adjustable control. The control is typically coupled to a potentiometer for setting a threshold signal which must be exceeded by the Doppler shifted input signal in order to obtain an output from the apparatus. Typically, the output is recorded.
A proper setting of the threshold signal requires knowledge of what the appearance of a proper signal should be. This is frequently not known to the operator; such knowledge being gained only by extensive use and operating experience. Moreover, the noise characteristics inherent in different Doppler probes which may be used with a particular instrument may be and often are different, thereby requiring knowledge of what the appearance of a proper output signal would be with each of the Doppler probes used. If the threshold signal is set too high, valuable information concerning the blood circulation or other condition under investigation is not recorded and is therefore lost. If the threshold signal is set too low, excessive noise is recorded with the signals from the condition under investigation which tends to mask the signals under investigation, thereby also causing the useful information to be lost.
Experience has shown that the threshold adjustment for a zero-crossing detector FM demodulator of the type employed in the present invention is baffling to the typical user and that the performance of the circuit depends critically on its proper setting especially when the signal-to-noise ratio is low.